This invention is directed to cookware formed from a laminate, said laminate comprising at least three sheets made from a thermoplastic resin, the inside sheet made from a thermoplastic resin having a higher use temperature than the thermoplastic resin the two outside sheets are made from. Also, this invention is directed to a laminate suitable for molding into cookware.
Cookware utilized in conventional ovens should have the capability of withstanding the great temperature variations existing between the temperature setting devices and the actual temperatures within the oven. Though the cookware is only exposed to the oven's actual temperature, the user's expectations of the cookware's capacity to withstand heat is a critical factor in the use of that cookware. Putting cookware that deforms at e.g. 200.degree. F. into an oven set for 325.degree. F. is clearly illogical. Equally illogical would be the use of the same cookware in an oven whose temperature setting device fails to accurately control the oven's temperature. Thus a low temperature setting could result in a high oven temperature, and the cookware would still deform. The realities of life are that few commercially available gas and electric ovens have accurate temperature controls and in most cases the ovens run hotter than the temperature setting. In a number of cases, an oven temperature setting of 400.degree. F. resulted in an oven temperature as high as 475.degree.-500.degree. F. This is the basis for the first sentence of this paragraph.
Plastics are typically termed thermoplastic or thermosetting. Thermoplastics are deformable with application of sufficient heat. Because thermosetting plastics (resins) are crosslinked, they are fairly resistent to heat deformation, certainly more so than thermoplastics. Consequently, thermosetting resins have been extensively used for cookware. For example, cookware have been made from melamine-formaldehyde resins, unsaturated polyester resins, and the like. Such plastics have excellent heat resistance. However, they do suffer from a number of significant deficiencies. Because they crosslink during their curing processes when molded, they shrink and pull away from the mold surfaces. Unless they are properly filled with small particulate fillers, the molded objects have very uneven surfaces, and they are subject to significant crazing and/or cracking. High filler loading adversely affects the physical properties of the molded object and precludes the direct obtaining of a glossy surface. Thermosetting resins are difficult to mold. They generally have to be compression or transfer-molded. Such processes require much materials handling, large equipment, complicated and relatively expensive molds, and significant energy costs.
Thermoplastics have been used for coating paper dishware and some of them have been used as cookware. However, their use as cookware is severely restricted, certainly to low temperature or microwave oven applications. Thermoplastics, such as Udel.TM. polysulfone (made by Union Carbide Corporation), have been sold for use in making cookware designed for microwave oven applications. One would expect that some of such cookware has been generally employed in conventional ovens as well. However, Udel.TM. polysulfone has not proven to be suitable for the wide temperatures used in conventional oven cooking and hence, its usage in such applications has not been recommended.
Thermoformed polyethylene terephthalate is used for cookware in microwave and conventional oven units, but is generally limited in use to about 350.degree. F. Above this temperature, the modulus of the material drops rapidly so that cookware will sag and distort and will be unstable from a handling standpoint when removing from the oven with a food load present in the container. In the 400.degree. F. range, the polyethylene terephthalate containers will distort severely and lose their shape.
Though the physical properties of a thermoplastic might be considered at first blush to be the basis for its use as generally employable cookware, i.e., cookware usable in any kind of oven up to a temperature of 500.degree. F., such is clearly not the case. Since cookware is in contact with the food placed therein, the plastic it is made from must be safe to use and not contaminate the food it contacts. Temperature gradients exist within conventional ovens and such a variable requires actual working information about a plastic's performance as cookware under a wide variety of conditions. Further, unless the cookware is intended to be disposable after the first use, it should have the capacity of withstanding repeated washings, by hand or by machine. It should be detergent resistant and not absorb food, oils and fats. It should be able to withstand warping on usage. If it is intended for household use, then it should meet the aesthetics typically favored, such as high gloss and smooth surfaces. Further, it is desirable that the thermoplastic be moldable into a variety of cookware configurations by a simple molding process such as vacuum forming or injection molding. Moreover, since the use conditions are quite severe, necessitating the use of a high performance plastic that tends to be more costly, then all of such performance capabilities are desirably achievable with the minimum amount of plastic usage.
U.S. Pat. No. 4,503,168 filed in the name of T. F. Hartsing on May 25, 1983, titled "Cookware Made From Polyarylethersulfone" (commonly assigned) describes cookware made from a composition comprising a polyarylethersulfone as the sole polymeric component, or when blended with other polymer(s), the polyarylethersulfone constitutes greater than 30 weight percent, said weight percent based on the weight of the polymeric materials in the composition. Also, said U.S. Pat. No. 4,503,168 describes cookware made from a composition containing the polyarylethersulfone as having a good combination of physical properties.
In the present invention it has been found that a particular laminate possesses the necessary combination of properties that a thermoplastic material need possess to be acceptable for cookware. Also the laminate provides extremely attractive and useful cookware which can be used in essentially all cooking oven applications.